Important properties in pearlescent pigments as a substrate are particle size, shape, surface property and refractive index. That is, uniform particle size is necessary to achieve clear colors because the ratio of reflected light to transmitted light are different between large-sized particles and small-sized particles. Further, the particle size is closely related to the wavelength of light, and greatly affects the tinting strength of pearlescent pigments.
Surface area increases with the decrease of the particle size, thereby increasing tinting strength and reflectivity and thus making color clear. However, it is difficult to uniformly coat small-sized particles with a metal or metal oxide, and the small size may decrease the effect of light interference and deteriorate the pearl gloss. Therefore, particles are preferred to have sufficiently uniform size for expression of various pearl gray colors.
The substrate in pearlescent pigments should also be transparent particles that have uniform thickness and smooth surface because these properties affect the expression of pearl gray color such as tinting strength and hiding power. When the thickness of particles is not uniform or surface is not smooth, most light reflects or scatters on the surface. Agglomeration of particles and non-uniform thickness also inhibit the expression of various pearl gray colors when the particles are coated with a metal or metal oxide.
Therefore, to be useful as a substrate for pearlescent pigments, flaky alpha-alumina crystals should be transparent flaky particles that have a thickness of 0.5 μm or less; an average particle size of 15-25 μm, while having a very smooth surface without causing agglomeration.
Examples of the conventional substrate of pearlescent pigments include lead carbonate, bismuth oxychloride (BiOCl) and natural or synthetic mica. The mica is also used as a reinforcing agent for improving the ductility and mechanical property of ceramic material or as an additive for improving thermal conductivity.
Meanwhile, the flaky alumina substrate has been conventionally prepared by using a hydrothermal method or by using titanium dioxide as additives for being used as a substrate of pearlescent pigments.
The hydrothermal method has problems of small particle size and low large aspect ratio. The latter comprises no aging process and has problems of small large aspect ratio and non-uniform particle size and dispersity despite superior properties in a substrate of pearlescent pigments.
Therefore, aluminum solution is hydrolyzed into pseudo-boehmite normally according to solution chemistry. The pseudo-boehmite undergoes phase transition into gamma-alumina (γ-Al2O3) at above 400° C. Hexagonal flaky crystals may be formed by changing the gamma-alumina (γ-Al2O3) into alpha-alumina in a molten salt solution by means of heat treatment to 850° C.
The flaky alpha-alumina crystals should have a thin and uniform plane, a large aspect ratio (=diameter/thickness) of 50 or higher along with transparency so that it may show a pearlescent gloss effect of various colors when coated with a metal or metal oxide.
However, the conventional flaky crystals have a relatively low large aspect ratio, and thus show unsatisfactory gloss caused by interference colors when coated with a metal and metal oxide.
The present inventors have filed a patent application directed to flaky alpha-alumina crystals comprising aluminum oxide and zinc oxide as an essential ingredient and a method of their preparation as a result of search on flaky alpha-alumina crystals with large aspect ratio [Publication of Korean patent application No. 2005-25126]. Thus prepared flaky alpha-alumina crystals have a thickness of 0.1-0.5 μm, a diameter of 15-25 μm and a large aspect ratio of 50-250.